Systems and methods for providing mirrors with high stiffness and low inertia involving chemical etching

ABSTRACT

A method is disclosed of fabricating a mirror for use in limited rotation motor systems, said method comprising the steps of providing a mirror structure including at least one wall section, and exposing the at least one wall section to a fluid etching agent to thereby provide chemical milling of the mirror structure.

PRIORITY

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 61/513,274 filed Jul. 29, 2011, the disclosure ofwhich is hereby incorporated by reference in its entirety.

BACKGROUND

The present invention generally relates to the fabrication of opticalcomponents in limited rotation motor systems, and relates in particular,to the fabrication of such components having high stiffness and lowinertia.

Scanning mirror systems generally include a mirror surface that iseither mounted on a substrate or is formed as part of a substrate,wherein the substrate is coupled to a shaft of a motor system. Suchmotors may either run continuously (e.g., for use with a polygonalmirror) or such motors may be limited rotation motors providing movementwithin a limited angular range. These applications require that themirror be as stiff as possible consistent with a minimum of inertia asthey are accelerated back and forth over the limited angular range.

In typical limited rotation scanning systems (also called galvanometersystems) a mirror is mounted to the output shaft of a limited rotationmotor and the limited rotation motor is controlled by a control loopthat seeks to cause the rotor of the motor, and therefore the mirror, tofollow a position and velocity command waveform with arbitrarily highfidelity.

There are limits, however, on the fidelity with which the system mayfollow the command. For example, the acceleration of the mirror withinthe system is limited by the rate of rise of current in the motorwindings. The positional precision is limited by the signal to noiseratio of the feedback method. The bandwidth of the system (which is itsability to move from position A to position B at a desired high velocityand to then settle at position B precisely in the shortest possibletime), is limited primarily by vibrations in the moving parts. Thebandwidth of the system will nominally be ½ the first torsionalresonance in the moving structure.

It is customary, therefore, to make the moving parts as stiff aspossible within the constraints of the allowable system inertia. Sincethe torque required of the motor to reach a specified acceleration isdirectly proportional to the inertia and is proportional to the current(whose rate of rise is limited as noted above), it is often the casethat when the system parameters are optimized for a particular inertia,some component, typically the mirror, even when made of a very highstiffness-to-inertia material, is not as stiff as is required to reachsystem bandwidth goals. In this case, extra material is added to themirror to increase its stiffness, but, at the cost of additionalinertia, requiring a larger, more expensive motor as well as a controlloop that is capable of driving the additional inertia.

There is a need therefore, for a limited rotation motor system thatprovides improved bandwidth without requiring a larger, more expensivemotor and accompanying control system.

SUMMARY

In accordance with an embodiment, the invention provides a method offabricating a mirror for use in limited rotation motor systems. Theincludes the steps of providing a mirror structure including at leastone wall section, and exposing the at least one wall section to a fluidetching agent to thereby provide chemical milling of the mirrorstructure.

In accordance with another embodiment, the invention provides a mirrorfor use in a limited rotation motor system, and the mirror includes abacking structure opposite a front of the mirror. The backing structureincludes at least one wall section having a tapered shape that tapers asthe wall extends away from a front of the mirror.

In accordance with a further embodiment, the invention provides a mirrorfor use in a limited rotation motor system that includes a backingstructure opposite a front of the mirror, and the backing structureincludes features that provide rigidity to the mirror and that havethicknesses that become reduced in a direction away from a front of themirror.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description may be further understood with reference tothe accompanying drawings in which:

FIG. 1 shows an illustrated diagrammatic enlarged view of a portion of amirror substrate to be processed in accordance with the presentinvention;

FIG. 2 shows an illustrated diagrammatic enlarged view of a portion of amirror substrate following processing in accordance an embodiment of thepresent invention;

FIG. 3 shows an illustrative diagrammatic front view of a mirrorsubstrate to be processed in accordance with an embodiment of thepresent invention;

FIG. 4 shows an illustrative diagrammatic top view of the mirrorsubstrate shown in FIG. 3 taken along line 4-4 thereof;

FIG. 5 shows an illustrative diagrammatic side sectional view of themirror substrate shown in FIG. 3 taken along line 5-5 thereof;

FIG. 6 shows an illustrative diagrammatic back view of the mirrorsubstrate shown in FIG. 3;

FIG. 7 shows an illustrative diagrammatic enlarged view of a portion ofthe back of the mirror substrate shown in FIG. 6;

FIGS. 8A-8C show illustrative diagrammatic views of a portion of amirror substrate following processing in accordance an embodiment of thepresent invention;

FIG. 9 shows an illustrative diagrammatic back view of a mirrorsubstrate that has been processed in accordance with an embodiment ofthe present invention;

FIG. 10 shows an illustrative diagrammatic enlarged view of a portion ofthe back of the mirror substrate shown in FIG. 8;

FIG. 11 an illustrative diagrammatic side view of the mirror substrateshown in FIG. 9 taken along line 11-11 thereof;

FIG. 12 shows an illustrative diagrammatic isometric view of the mirrorsubstrate of FIG. 8; and

FIG. 13 shows an illustrative diagrammatic view of a limited rotationmotor system using an optical element that includes a mirror substrateof the present invention.

The drawings are shown for illustrative purposes only.

DETAILED DESCRIPTION

Certain high stiffness and low inertia materials are known to be usedfor making moving (or dynamic) mirrors that move during use, such asmirrors in limited rotation motor systems. It is desired that suchmirrors (ideally) have infinite stiffness and zero inertia, andberyllium for example, provides an excellent choice given its highstiffness and low mass.

It is also known that mirror structures may be machined to furtherreduce the mass of the mirror, specifically on the back of the mirrorand near the edges furthest from the axis of rotation of the mirror.This machining is designed to further reduce the mass of the mirrorwhile not significantly reducing the stiffness of the mirror. In thisregard, it is desired to machine the mirror in such a way that wallsremain to provide stiffness (e.g., in a honeycomb pattern) that are asthin as possible.

While materials used for mirror structures have very low specificinertia (gm-cm² per unit area), it is difficult to machine certain lowinertia materials such as beryllium to very fine thicknesses withoutcracking. Beryllium is also expensive to machine and produces a dustthat is hazardous. Machine tooling is generally unique to specificmaterials, requiring various speeds, feeds, lubricants, coolants, toolgeometries materials and coatings. It is therefore desirable to increasethe manufacturing speed of beryllium mirrors and at the same timeovercome the limitations of the known machining art.

FIG. 1 shows at 10 a beryllium mirror substrate, including wall sections12 and a floor 14 that remain following material removal. As is know inthe art, a computer file representing a solid model may be converteddirectly into machining instructions on a machine tool that mills thepart in plan, drills the longitudinal hole, and performs such secondaryoperations such as drilling and tapping holes as may be required whilestill a section of a surface of the parent beryllium block.

The substrate 10 may be further processed by removing it from the parentmaterial block on which it is milled by means of a sawing-off processsuch as wire EDM (Electrical Discharge Machining) or electro chemicalsawing. The separated substrate is then finished typically in accordancewith desired tolerances. It may then conventionally be used as-is, orfurther processed by plating, vacuum coating, or both.

The present invention is directed to a process for generating thesubstrate itself. As will be obvious to those skilled in the machiningart, the production of a single substratum on a face of a parent blockor the production of a multiplicity of substrata on one or more faces ofa parent block or in fact the production of a single substratum from asingle near-net-shape block all share the same issues and thereforethese variants do not depart from the spirit and scope of the invention.As will also be obvious to those skilled in the art, the exact size andshape of the substratum and the precise configuration of the stiffeningstructure on the back of the mirror are variants which do not departfrom the spirit and scope of the invention.

One of the difficulties in machining beryllium is that the surfacedevelops cracking as a result of the machining forces and the heatdeveloped. With care, using very sharp tools, flood coolant, and spindlespeeds in milling under 10,000 RPM these cracks are restricted to thetop 10 microns or so of the surface. Even so, they must be removed aftermachining and before use because otherwise they tend to grow in lengthand depth, particularly if the part is stressed as during acceleration,until they meet in the interior of the part and cause rupture.

Unfortunately, when the section thickness of the beryllium is severelyreduced (as it must be in order to produce the low inertia desired inmirror substrates) at some reduced thickness, section bending takesplace during machining. This bending causes deeper cracking. As aresult, the minimum section thickness practical has been approximately0.5 mm in structures of the scale of mirror substrates as shown at d₁ inFIG. 1. This section thickness then essentially puts a lower limit onthe inertia that can be achieved.

The surface cracking caused during machining such as milling may beremoved by immersing the clean part after machining in an etchantsolution such as 60% concentration hydrofluoric acid (HF) 1 part and 69%concentration nitric acid (HNO₃) 9 parts or other suitable etchant. Thematerial removal rate at 20 C+/−5 C is about 18 microns per minute persurface exposed to the etchant. Although this may seem slow comparedwith typical milling tooth loading of 5 microns at a spindle speed of10000 RPM, the milling takes place over a single line contact somewhereon the part, whereas the etching takes place simultaneously over theentire exposed surface of the part, and is therefore much faster.Because of the linear relationship between material removed andimmersion time, agitation bath composition and temperature remainingconstant, it is appropriate for the desired metal removal depth to becontrolled by the time the part is immersed in the bath.

In accordance with an embodiment of the invention, a beryllium mirrorstructure (such as shown in FIG. 1) may remain in the etching bath foran extended period of time of, for example, 5 to 6 minutes. Followingsuch a treatment, sections of the beryllium structure become etched tomuch smaller dimensions as shown at 20 in FIG. 2. The etched walls areshown at 22 and the etched floor is shown at 24. The thickness of thefloor section, for example, may be reduced from d₂ (e.g., about 0.5 mm)to d₄ (e.g., about 0.4 mm). Further, the wall sections become tapered,having triangular cross-sectional shapes. The thickness of the wallsections, for example, may be reduced from d₁ (again e.g., about 0.5 mm)to walls having a variable thickness that is for example., about 0.25 mmat the thickest part (d₃) down to possibly zero at the top. This has thedesirable effect of further reducing the mass of the ribbing andtherefore the inertia of the part, particularly since the more mass isremoved further from the axis of rotation of the mirror.

Mirrors formed of such a process were tested and found to beexceptionally low in specific inertia. During further development of theprocess, it was found that the ribs could be reliably reduced to a lineat the top without reducing their height (stiffness) simply bycontrolling the immersion time. The triangular shaping of the ribcross-section is attributed to partial exhaustion of the etchant insidethe closed cells.

Although the removal of material from the reflective face of thesubstrate reduces its inertia, it also reduces its stiffness: however, aminimum stiffness is required in order to support the forces producedduring polishing of the reflective surface. In keeping with therequirement that the reflective surface be flat to ¼ wavelength orbetter at the wavelength of use, it follows then that the requiredminimum section thickness (the inverse of stiffness first ordertherefore inertia all other things remaining constant) will vary withthe intended wavelength of use. Conventionally, it was necessary tomachine a mirror substratum to a section thickness in inverse proportionto the intended wavelength of use (¼ of a shorter wavelength is asmaller absolute allowed departure from flatness, and so requires astiffer substrate).

FIGS. 3-7 show a beryllium mirror structure 30 having a front side 32that provides a highly reflective surface and a back side 34. Theopening 33 may include mirror damping material as disclosed, forexample, in U.S. Patent Application Publication No. 2010/0271679, thedisclosure of which is hereby incorporated by reference in its entirety.As shown at 36 in FIGS. 6 and 7, the back side 34 is machined to removedberyllium material in a honeycomb pattern from areas 36, leaving wallsections 38.

With reference to FIGS. 9-12, following chemical treatment in the fluidetching agent (as shown in FIGS. 8A-8C), the wall sections 46 and thefloor 48 of the mirror structure 40 are etched to provide tapered shapesas shown in FIGS. 2 (at 22 and 24 respectively). The wall sections, inparticular, have a shape that tapers as the wall extends away from theaxis of rotation of the mirror. The front side 44 of the mirror 40remains unetched due to masking.

Because the etching process is fast and un-attended, the presentinvention is very economical compared with the machining of discreteindividual section thicknesses, so that even if in the future a methodis found which allows the direct machining of thinner sections thanthose now possible, processes of the invention will continue to providea more economical approach to the production of very thin very lowinertia mirrors. In practice therefore, a single rather thick substratumis milled or otherwise machined in multiple units, and the individualunits are then processed using etching to the final dimensions requiredfor a particular wavelength or wavelength interval of use.

In accordance with various embodiments of the invention, therefore, asection of beryllium mirror stiffening ribs and/or a face may be reducedto a desired thickness by etching, and/or by masking the exposed mirrorface to effectively deepen the ribs without causing cracking in thereflecting face. In accordance with further embodiments, a rate ofmechanical agitation may be controlled to control the partial exhaustionof the etchant so that triangular cross-sections thinner at the open endare produced. In accordance with further embodiments, selected areas ofthe mirror substrate may be masked prior to etching to prevent etchingin those areas, and in further embodiments, the etching substrata may bemachined to a uniform over-size section thickness to a variable finaldesired section thickness.

As further shown in FIG. 12, in a limited rotation motor system 50, theoptical element 52 (e.g., the mirror 40) is coupled to a limitedrotation motor 54 via mirror mounting structure 56 (e.g., via a clamp,threaded mounting structure and/or a tapered mounting structure asdisclosed for example in U.S. Pat. No. 7,212,325, the disclosure ofwhich is hereby incorporated by reference in its entirety) for rotationabout the motor rotor axis A_(R). The system 50 also includes a positiontransducer 58 that is coupled to a feedback control system 60 thatprovides a command signal 62 to the motor 54 responsive to an inputcommand signal from an input node 64 and a feedback signal 66 from theposition transducer 58 to control the speed and/or position of the motorshaft, and therefore the optical element 52.

The feedback control system is used to cause the rotor of the motor, andtherefore the mirror, to follow a position and velocity command waveformwith arbitrarily high fidelity. There are limits however, on thefidelity with which the system may follow the input command signal. Theacceleration of the mirror in the system for example, is limited by therate of rise of current in the motor windings, and the positionalprecision is limited by the signal to noise ratio of the feedbacksystem. The ability of the system to move the mirror from a position Ato a position B at a desired high velocity and to then settle atposition B precisely in the shortest time (the bandwidth of the system)is limited primarily by vibrations in the moving parts. Providing amirror substrate in accordance with the invention advantageously permitsthe mirror to be very high in stiffness yet low in inertia.

Those skilled in the art will appreciate that numerous modifications andvariations may be made to the above disclosed embodiments withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A method of fabricating a mirror for use inlimited rotation motor systems, said method comprising the steps ofproviding a mirror structure including at least one wall section,wherein said mirror substrate includes a beryllium mirror structure, andwherein said method includes the step of drawing the beryllium mirrorstructure from a fluid etching agent at a controlled rate such that theat least one wall section forms a tapered wall.
 2. (canceled)
 3. Themethod as claimed in claim 1, wherein said fluid etching agent includeshydrofluoric acid (HF).
 4. The method as claimed in claim 1, whereinsaid fluid etching agent includes nitric acid (HNO₃).
 5. The method asclaimed in claim 1, wherein said method includes the step of applying amask to a protected area to prevent etching of the protected area. 6.The method as claimed in claim 5, wherein said protected area is a frontreflective surface of the mirror.
 7. The method as claimed in claim 1,wherein said method further includes the step of agitating a bath thatincludes the beryllium structure and the fluid etching agent.
 8. Themethod as claimed in claim 7, wherein said method further includes thestep of controlling a rate of agitation of the bath that includes theberyllium and the fluid etching agent to control a shape of the at leastone wall section.
 9. The method as claimed in claim 1, wherein saidmirror structure is removed from the fluid etching agent when the mirrorreaches a target reduced weight.
 10. The method as claimed in claim 1,wherein said method further includes the step of drawing the berylliumstructure from the fluid etching agent such that the at least one wallsection forms a tapered wall.
 11. The method as claimed in claim 1,wherein said method provides uniform milling of exposed surfaces of theberyllium structure.
 12. A mirror for use in a limited rotation motorsystems formed from the method of claim
 1. 13. A mirror for use in alimited rotation motor system, said mirror comprising a backingstructure opposite a front of the mirror, said backing structureincluding at least one wall section having a tapered shape that tapersas the wall extends away from a front of the mirror.
 14. The mirror asclaimed in claim 13, wherein said backing structure includes beryllium.15. The mirror as claimed in claim 13, wherein said wall sections areformed by chemical etching.
 16. The mirror as claimed in claim 13,wherein said tapered shape of the wall section has a thickness of nogreater than about 0.25 mm.
 17. The mirror as claimed in claim 13,wherein said backing structure includes a plurality of hexagonallyshaped wall sections each of which is tapered as the wall extends awayfrom a front of the mirror.
 18. A mirror for use in a limited rotationmotor system, said mirror comprising a backing structure opposite afront of the mirror, said backing structure including features thatprovide rigidity to the mirror and that have thicknesses that becomereduced in a direction away from a front of the mirror.
 19. The mirroras claimed in claim 18, wherein said backing structure includesberyllium.
 20. The mirror as claimed in claim 18, wherein said featureshave thicknesses of no greater than about 0.25 mm.
 21. A method offabricating a mirror for use in limited rotation motor systems, saidmethod comprising the steps of providing a mirror unit including abacking structure that is opposite a mirror surface, applying a mask tothe mirror surface, and exposing the backing structure to a fluidetching agent to thereby provide chemical milling of the backingstructure.
 22. The method as claimed in claim 21, wherein said backingstructure includes beryllium.
 23. The method as claimed in claim 21,wherein said method further includes the step of agitating a bath of thefluid etching agent that includes the backing structure.
 24. The methodas claimed in claim 21, wherein said method further includes the step ofdrawing the backing structure from a bath of the fluid etching agent ata controlled rate such that the backing structure forms tapered walls.25. The method as claimed in claim 24, wherein the tapered walls have athickness of no greater than 0.25 mm.
 26. A method of fabricating amirror for use in limited rotation motor systems, said method comprisingthe steps of providing a mirror unit including a backing structure thatis opposite a mirror surface, exposing the backing structure to a fluidetching agent to thereby provide chemical milling of the backingstructure; and of drawing the backing structure from a bath of the fluidetching agent at a controlled rate such that the backing structure formstapered walls.
 27. The method as claimed in claim 26, wherein saidmethod further includes the step of agitating a bath of the fluidetching agent that includes the backing structure.